Allergen immunotherapy continues to be used worldwide in the management of allergic rhinitis and asthma. We continue to refine our knowledge of the indications for immunotherapy, the allergens that can be used successfully, and the requirements for dosage and route of administration. Several additional allergens have been shown to be effective in controlled studies. The sublingual route of administration is safe and can be effective if the dose is large enough. Since reviews in 1998, further studies of immunologic changes after immunotherapy have confirmed that T(H)2 lymphocyte-mediated responses are suppressed, and T(H)1 responses are stimulated. Eosinophil and basophil inflammatory responses to allergen exposure are downregulated through one or both of these mechanisms. Research continues to seek improvements through allergen vaccine modifications suggested by recent immunologic discoveries. None of these is available in practice.